fix parenthesis output of AST_ClassExpression (#2677)

fixes #2676

lib/output.js

@@ -730,15 +730,9 @@ function OutputStream(options) {
         return p instanceof AST_PropAccess && p.expression === this;
     });
 
-    PARENS(AST_ClassExpression, function(output){
-        return output.parent() instanceof AST_SimpleStatement;
-    });
-
     // same goes for an object literal, because otherwise it would be
     // interpreted as a block of code.
-    PARENS(AST_Object, function(output){
-        return first_in_statement(output);
-    });
+    PARENS([ AST_ClassExpression, AST_Object ], first_in_statement);
 
     PARENS(AST_Unary, function(output){
         var p = output.parent();

test/compress/harmony.js

@@ -1197,3 +1197,18 @@ class_method_using_arguments: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_2676: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        class A {}
+        A.a = 42;
+    }
+    expect: {
+        (class {}).a = 42;
+    }
+}